A heavy duty pneumatic radial tire generally comprises a pair of bead wires, a pair of side walls, and a plurality of belt layers disposed between a tread portion and a radial carcass layer.
In such a heavy duty radial tire, the radial carcass layer comprises at least one steel cord layer or organic fiber cord layer disposed at an angle of about 90.degree. to the circumferential direction of the tire, while the belt layer comprises at least three layers for retaining and supporting the high inner pressure and high load of the heavy duty tire. At least one layer among these three layers in the belt layer is a reinforcing layer, and at least two layers constitute a tension-resistant layer. The reinforcing cords of the tension-resistant layer cross each other at an angle of 15.degree. to 25.degree.. On the other hand, the reinforcing layer is disposed on the outside of the tension-resistant layer (on the side of the ground-contacting portion of the tread) and has a width smaller than that of the tension-resistant layer. Further, the reinforcing layer has a relatively small cord angle to the circumferential direction of the tire and is disposed so as to cross or parallel the cords constituting the tension-resistant layer.
FIGS. 3 and 4 are a meridian half-crosssectional view of the above-described conventional pneumatic radial tire and a plain exploded view of the belt portion thereof, respectively.
In the drawings, a belt layer is laminated and disposed between a tread portion 2 and a carcass layer 3 of a tire 1. The belt layer comprises a belt reinforcing layer V.sub.1 disposed adjacently to the tread portion 2 and having a cord angle of 15.degree. to 25.degree. between the cord direction and the circumferential direction (A--A' line) of the tire (i.e., the first layer disposed from the tread portion towards the carcass layer) and tension-resistant layers V.sub.2 and V.sub.3 for a belt portion 4 having a structure of at least two layers (i.e., the second and third layers disposed from the tread portion towards the carcass layer). The tension-resistant layers are laminated beneath the belt reinforcing layer V.sub.1 so as to parallel or cross it and cross each other at a cord angle of 15.degree. to 25.degree. to the circumferential direction (A--A' line) of the tire.
Another known tire has a belt portion 4 in which an auxiliary belt layer V.sub.4 having a cord angle of 40.degree. to 75.degree. between the cord direction and the circumferential direction (A--A' line) of the tire is additionally lamination-disposed between the above-described carcass layer 3 and the tension-resistant layers V.sub.2 and V.sub.3 of the belt portion 4.
In these radial tires, the carcass layer 3 has a single- or multi-layer structure, and the carcass cords have an angle of about 90.degree. between the cord direction and the circumferential direction of the tire (i.e., substantially radially).
In the belt portion 4 of the above-described conventional tires, particularly the first to third belt layers, i.e., belt tension-resistant layers V.sub.1 V.sub.2, and V.sub.3 in FIGS. 3 and 4 are made of cords disposed at the same angle from the standpoint of productivity. As shown in FIG. 4, the second belt layer V.sub.2 and the third belt layer V.sub.3 were disposed so as to cross each other at the same cord angle although the directions of the disposition of the cords are different from each other, while the first layer V.sub.1 was disposed on the second layer V.sub.2 in the same direction (or so as to cross each other) at the same belt angle as that of the second belt layer V.sub.2 and the third belt layer V.sub.3.
However, the conventional tires having the above-described structure were unsatisfactory with respect to the driving stability under heavy duty conditions because they are aiming principally at ensuring the durability of the belt portion during high speed running.